1. Field of the Invention
The present invention relates to an organic electroluminescent device, a method for manufacturing an organic electroluminescent device, an image display device, and a method for manufacturing an image display device.
2. Description of the Related Art
In recent years, an organic electroluminescent element (hereinafter referred to as an organic EL element) has been developed. The organic EL element is a luminescent element which can emit high-intensity light by a direct current low voltage driving. The organic EL element is configured in a simple manner. In particular, the organic EL element includes two electrodes and a hole transport layer or an organic luminescent layer. The two electrodes face one another. The hole transport layer includes a hole transport material provided between the two electrodes. The organic luminescent layer includes an organic luminescent material. According to this organic EL element, an electrical current is applied between the two electrodes. As a result, the organic luminescent layer emits light. This emitted light is taken out by an optically-transparent electrode.
According to the organic EL element structured as described above, a configuration is also possible in which both electrodes are placed at both sides of the luminescent layer. However, in order to achieve increased efficiency in emitting light, it is often the case that an injection layer or a transport layer is provided, or, alternatively, an injection layer and a transport layer are both provided. An example is a structure in which a hole injection layer or a hole transport layer is provided between an anode and a luminescent layer, or, alternatively, a hole injection layer and a hole transport layer are both provided between a positive electrode and a luminescent layer. Another example is a structure in which an electron injection layer or an electron transport layer is provided between a negative electrode and a luminescent layer. In the technical field of organic EL elements, a luminescent medium layer refers to the entire structural body including the plurality of film layers sandwiched between both electrodes.
In general, the type of organic EL elements is divided into two types according to the organic luminescent material used in the organic luminescent layer. The first type is an organic EL element using a low-molecular organic luminescent material (hereinafter referred to as a low-molecular organic EL element). The second type is an organic EL element using a polymer organic luminescent material (hereinafter referred to as a polymer organic EL element).
In general, according to a method of forming a low-molecular organic EL element, a thin film is formed using a dry coating method such as a vacuum deposition method and the like. According to such a method of forming a low-molecular organic EL element, when a patterning of the hole transport layer or the organic luminescent layer is necessary, a metal mask and the like is used to form a layer having a pattern according to an opening part of the mask. However, there is a problem with such a patterning method in that, as the area of the substrate increases, it becomes more difficult to obtain a desired level of precision in the patterning. In addition, since a film is formed in a vacuum, there is a problem in that a throughput is low. Furthermore, there is likely to be greater influence due to an irregular form included in a foundation part located below the hole transport layer or the organic luminescent layer. As a result, an unevenness may appear in an image shown by an image display device including the hole transport layer or the organic luminescent layer.
According to a method of forming a polymer organic LE element, a coating liquid is prepared such that an organic luminescent material is dissolved. Then, a wet coating method is used to apply a coating liquid on a substrate. In this way, a thin film is formed. Examples of the wet coating method for forming a thin film include a spin coating method, a bar coating method, a protrusion coating method, a deep coating method, and the like. However, when these wet coating methods are used, it is difficult to perform a patterning with a thin film at a high level of precision. It is also difficult to form a thin film by applying three colors including R, G, and B individually. Therefore, among the methods for forming a polymer organic EL element, it is believed to be most effective to use the printing method to form a thin film. The printing method allows a patterning to be performed while applying a plurality of materials individually.
In addition, when various types of printing methods are performed, a glass substrate is often used as the substrate for organic EL elements or displays. Therefore, a method using a hard block such as a metallic printing block, like the gravure printing method, is not suitable as a method for forming a polymer organic EL element. Meanwhile, the offset printing method, using an elastic rubber blanket, is suitable for forming a polymer organic EL element. In addition, the typography method, using a similarly elastic rubber plate or a photopolymer printing, is also suitable for forming a polymer organic EL element. In fact, as an attempt to use these printing methods, a method based on the offset printing method (Japanese Unexamined Patent Application, First Publication No. 2001-93668), a method based on typography (Japanese Unexamined Patent Application, First Publication No. 2001-155858), and the like are suggested.
According to a polymer organic EL element, a hole transport layer is provided to obtain an emission of light by applying a low voltage. A low-molecular organic material and a polymer organic material are normally used as the material used for this hole transport layer. This is similar to the case regarding the luminescent layer.
TPD (tri-phenylene-amin type derivative: see Japanese Patent No. 2916098) is known to be a representative example of a low-molecular hole transport material. A PEDOT:PSS (a compound of polythiophene and polystyrene sulfonic acid: see Japanese Patent No. 2851185) is known to be a representative example of a polymer hole transport material. A dry coating method or a wet coating method is used as the film forming method. This is similar to the case regarding the organic luminescent layer.
Incidentally, an attempt has also been made to use an inorganic material as the hole transport material (see Japanese Unexamined Patent Application, First Publication No. H9-63771). This Japanese Unexamined Patent Application, First Publication No. H9-63771 suggests that the durability of an organic EL element, using an organic compound such as the TPD or the PEDOT:PSS etc. as a hole transport material, is low. However, speaking of the luminescent characteristics of the organic EL element, an emission with a high degree of brightness and a high level of efficiency is obtained as an initial characteristic (hereinafter referred to as an initial characteristic). Thus, a driving with a direct current and a low voltage is achieved. However, the brightness or the efficiency declines due to the driving condition being continued or preserved for a long period of time. As a result, the luminescent characteristics are observed to decline.
The durability of the organic EL element is low because the heat resistant property of the thin film including TPD, which is a low molecular organic compound material, is low. The durability of the organic EL element is low also because the molecules within the non-crystallized thin film are altered due to the application of electrical current and becomes a microcrystallite, thereby reducing the functionality. Meanwhile, when a polymer material PEDOT:PSS is used, the functionality is believed to decrease because the ion element is scattered to the electrode or the organic luminescent layer and the like, when electric current is applied to the organic layer. The inorganic material includes a lot of material that has a superior heat resistance characteristics and has a high degree of electrochemical stability. Accordingly, this problem may be resolved by using an inorganic material as the hole transport material.
FIG. 20A is a diagram showing a structure of a general organic EL element. A first electrode 102 is formed on the substrate 101. A hole transport layer 104, an organic luminescent layer 106, and a second electrode 107 are stacked on the first electrode 102 in series. The hole transport layer is required to have heat resistance or electrochemical stability. Further, the hole transport layer is also required to have light permeability, a high work function, and resistance to the environment. However, the characteristic that is most required of the hole transport layer is to have a high degree of charge injection.
Work function, band gap, specific resistance, and the like affect the degree of charge injection. Each of these characteristics may be adjusted by selecting the inorganic material used as the hole transport layer. However, it is difficult to satisfy all of the characteristics that are required. Even if a material satisfying the required characteristics of work function and band gap is used, a problem occurs as described below, if the specific resistance is low.
FIG. 20B is a cross sectional view of a general organic EL display device. The layering structure of the organic EL element is similar to what is shown in FIG. 20A. However, in order to configure an image display device displaying three colors of RGB, a partition wall 203 is provided. The partition wall 203 partitions off pixels. A hole transport layer 104, an organic luminescent layer 106, and a second electrode 107 are formed so as to cover a first electrode 102 and the partition wall 203. In order to drive such an image display device, it is necessary to control the luminescence of each pixel.
According to an electroluminescence of a normal organic EL element, a hole is injected from a direction opposite to a surface at which a substrate contacts a first electrode. Then, the hole moves inside the hole transport layer so that the travel path of the hole is perpendicular to the first electrode. Furthermore, an electron injected from a negative electrode is recombined with a hole within the organic luminescent layer. In this way, emitted light is obtained.
In FIG. 20B, when the specific resistance in the film-thickness direction of the hole transport layer 104 is referred to as a perpendicular specific resistance, and when the specific resistance in the film-surface direction of the hole transport layer 104 (a direction horizontal to the surface of the hole transport layer 104) is referred to as a horizontal specific resistance, a normal electroluminescent light is obtained if the perpendicular specific resistance is lower than the horizontal specific resistance. However, when the perpendicular specific resistance is higher than the horizontal specific resistance, an electric current that does not contribute to the luminescent phenomenon (hereinafter referred to as a leak current) runs in the film-surface direction. The hole moves within the hole transport layer 104 formed on the partition wall 203. Thus, the characteristic of the organic EL element declines. Furthermore, according to an image display device configured with an organic EL element as described above, there is a problem in that, when a leak current runs between adjacent pixels, it becomes difficult to control the display in a desirable manner.
The present invention is made according to the problems described above. A first object of the present invention is to provide an organic electroluminescent device and a method for manufacturing an organic electroluminescent device, which can be easily manufactured and which can reduce or restrain a leak current. The organic electroluminescent device includes a predetermined luminescent medium layer formed on the entire display region. This display region includes the partition wall which partitions off pixels.
In addition, a second object of the present invention is to provide an organic electroluminescent device, an image display device, and a method for manufacturing an image display device. The organic electroluminescent device is configured so that an organic EL element is used. The organic EL element includes a hole transport layer using an inorganic material. Thus, a leak current in the film-surface direction of the hole transport layer is reduced. Furthermore, element characteristics are enhanced.